This invention relates generally to cable television systems and particularly to cable television systems employing coded signals for special services.
Most cable television operators offer different classes or types of service to their subscribers for additional fees. For example, in addition to conventional network television programs, the cable operator may provide one or more channels dedicated to particular subjects or uses such as movies, sports, special events, and the like. Most cable systems either include or have access to a central computer that automatically identifies each subscriber equipment and the class of service for which the subscriber has contracted. They usually include provision for automatic billing and for determining whether the subscriber is current in his payments. The subscriber equipment is periodically interrogated to update information in the subscriber equipment memory to control suitable decoding circuitry for enabling reception of the special programming for which he has subscribed.
In one well known system, the television signal vertical blanking interval is used to carry instruction and control information to the subscriber equipment. Specifically, data is transmitted on groups of successive horizontal lines in the vertical blanking interval. The data transmission is recognized by circuitry in the subscriber equipment, detected and used to identify the subscriber equipment, load its memory, verify information therein, etc. On one of the horizontal lines, a "market code" is sent which, together with an "address code" on another line, uniquely defines the particular subscriber equipment. The market code generally denotes the geographical area in which the subscriber equipment is located. The point of interest for this invention is that the market code is sent in each field of the transmitted signal and it does not change.
It will be appreciated that a cable operator has a sizable investment in subscriber equipment. As improved equipment is designed and built, it is important that it be compatible with existing equipment. This requirement doesn't post significant difficulty except in the area of the type of encoding/decoding system employed. Signal encoding or scrambling is very important because of the growing need to guard against signal "piracy" and the like. Some systems are easier to defeat, that is to design pirate decoding equipment for, than others. It may very well be that a cable operator would like to update his equipment with that operating on a more sophisticated encoding system, for example. An "overnight" replacement of all of the subscriber equipment is, of course, impossible. Even if such a transition could be made, its cost would be prohibitive. A gradual transition may be made, but only at great inconvenience and at substantially added cost by, in essence, having two different systems and duplicating the special programming on different television channels. The system would then operate with two different encoding systems and accommodate both the old equipment and the new equipment. Such an arrangement is also not practical nor desirable. Accordingly, there is a need in the art to provide a system for enabling different types of encoded transmissions to be received by subscribers.